Quillworts

Order Isoëtales

The quillworts are a group of grass-like lycophytes in the genus Isoëtes that have a deep ancestry. Modern forms are small with linear leaves living in nutrient-poor lakes and ponds. In order to identify species, spores (and a microscope) are needed. They have an underground corm, which produces tiny amounts of wood, and roots that can absorb carbon from the soil (unlike any other plant in the world). They are also able to switch between C3 photosynthesis and CAM photosynthesis, and may represent the first plants to create CAM photosynthesis.

Quillworts are cousins to the enormous scale trees of the Carboniferous coal age, retaining some of the features of this group. Taxa resembling Isoëtes date back to the Triassic Period.

Above: Isoëtes growing on the edge of an oligotrophic lake

Above: Close-up of Isoëtes growing in shallow water

Classification

└Embryophytes

└Tracheophytes

└Lycophytes

└Lycopsida

└Isoëtales

Diversity

The Isoëtales is represented by one extant family (Isoëtaceae) and three extinct families: Chaloneriaceae †, Pleuromeiaceae †, and Nathorstianaceae †
Single living genus: Isoëtes (pronounced "I-so-ate-ees") with over 200 spp. found throughout the world, although they tend to be rare in local environments

Geologic range

Rhizomorphic lycophytes, which include extinct isoetids and lepidodendrid scale trees, date back to the Late Devonian.
Plants resembling the grass-like quillworts, e.g., Pleuromeiaceae, date back to the Triassic Period.

Sporophyte (=spore-producing phase)

Vegetative features

Stem

Brown, lobed, corm-like stem called a rhizomorph
Tiny amounts of wood (secondary xylem) and bark (phellem) are produced inside the rhizomorph
- **Quillworts are the only living spore-bearing plants with secondary growth**

Leaves

Tufted, grass-like microphylls / lycophylls
Each leaf is also a sporangium-bearing leaf (sporophyll)
Sporangium with a ligule
Some species of Isoëtes are capable of both C3 photosynthesis and CAM photosynthesis (Wickell, 2021).
- When terrestrial, these plants absorb and fix carbon dioxide as C3 plants
- When aquatic, these plants use CAM photosynthesis to concentrate CO2, since they are living in water with low amounts of carbon dioxide or bicarbonate

Roots

Isoëtes possesses true roots from the rhizomorph; they are not leaf homologs as sometimes interpreted in the past (Yi & Kato, 2001; Hetherington et al., 2020)
Roots branch dichotomously after they emerge from the rhizomorph
Roots have an air canal running through the root, similar to other wetland plants
Isoëtes can absorb carbon through roots (**unique among plants**)

Reproductive features

Spore cases (sporangia)

Sporangia are adaxial on sporophylls, like other lycopods
Each leaf of the plant is a sporophyll, with sporangia located at the base
Isoëtes is heterosporous with some leaves being megasporophylls (with a megasporangium) and others are microsporophylls (with a microsporangium)

Gametophyte (=gamete-bearing phase)

Small, but multicellular gametophytes
Mega- and micro-gametophytes are endosporic (not released from spore)
They are also heterotrophic and not photosynthetic
- Supply of nutrients is limited to what is contained in the original spore.
- Female gametophyte is larger and slightly emerges to expose an archegonium and rhizoids
- Male gametophyte is completely retained within the microspore, and releases sperm, which swim to the female gametophyte for fertilization

Above: Isoëtes growing in a wetland

Above: Longitudinal section of Isoëtes showing the corm (A), the roots (B), young micro-sporangium (C), and mature mega-sporangium (D)

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